The basic idea of hierarchical modulation technology is to divide the used constellation points into layers such as base layer and enhancement layer. A user equipment with poor channel quality or low priority can only obtain the information contained in the constellation points of the base layer by demodulation, while a user equipment with good channel quality or high priority can obtain the information contained in the constellation points of the enhancement layer by demodulation.
The basic principle of hierarchical modulation is explained through an example in which the “base data stream” is transmitted in the conventional Quadrature Phase Shift Keying (PQSK) modulation mode, and the “enhancement data stream” is transmitted in the 16 Quadrature Amplitude Modulation (16 QAM) mode. FIG. 1 illustrates a schematic diagram of hierarchical modulation. In FIG. 1, “•” is the constellation point of 16 QAM, and “x” is the constellation point of QPSK. If the constellation point of QPSK is assumed as a large constellation point (as shown by the dashed line circle in FIG. 1), four constellation points within the same quadrant among the constellation points of 16 QAM may be deemed as small constellation points inside the large constellation point. In this mapping mode, a user equipment with poor channel quality can only identify the “large constellation point” to thereby obtain the base data stream by demodulation, due to a weak identification capability. However, a user equipment with good channel quality can identify both the “large constellation point” and the “small constellation point” inside thereof, due to a strong identification capability.
Thus, when a certain symbol is received, the user equipment with poor channel quality can only obtains a two-bit data by demodulation, while of the user equipment with good channel quality can obtain a four-bit data by demodulation. As illustrated in FIG. 1, when a certain symbol is received, the user equipment with poor channel quality can only identify that the symbol is located at a “large constellation point” in the fourth quadrant and only obtain bit data of “01” by demodulation, while of the user equipment with good channel quality can identify that the symbol is not only located at a “large constellation point” in the fourth quadrant, but also located at a “small constellation point” on the upper left corner of the “large constellation point”, and then can obtain data of “1001” by demodulation. That is, the user equipment with poor channel quality can only obtain the anterior two bits of “1001” by demodulation. In general, the anterior two bits corresponding to the QPSK modulation, which has a lower order than the 16 QAM, in the 16 QAM constellation point are referred to as most significant bits (MSBs), and the posterior two bits are referred to as least significant bits (LSBs).
Next, the resource mapping methods for multiple user equipments when adopting the hierarchical modulation and the conventional scheduling are explained with reference to FIGS. 2, 3A and 3B. FIG. 2 illustrates a communication model in which a cell of the base station 1 has two user equipments, wherein the channel quality of a first user equipment UE1 is far better than that of a second user equipment UE2. FIG. 3A illustrates a resource mapping method in the conventional scheduling, and FIG. 3B illustrates a resource mapping method when the hierarchical modulation is adopted. As illustrated in FIG. 3A, in the conventional scheduling, a base station allocates orthogonal resources to two user equipments, and the orthogonal resources may be code-word orthogonal, time-frequency orthogonal, etc. For example, in case of time-frequency orthogonal resources, a first user equipment UE1 is allocated a time-frequency resource 1, and a second user equipment UE2 is allocated a time-frequency resource 2 orthogonal to the time-frequency resource 1. As illustrated in FIG. 3B, when the hierarchical modulation is adopted, the first user equipment UE1 and the second user equipment UE2 will occupy the time-frequency resources 1 and 2 simultaneously. In order to improve throughput and coverage of the cell, the same modulation mode is adopted for the time-frequency resources 1 and 2. Further, bits of the data of the second user equipment UE2 are mapped to the MSBs of the constellation points, and bits of the data of the first user equipment UE1 are mapped to the LSBs of the constellation points. As a result, the throughput and coverage of the cell can be improved.
Next, the bit mapping conditions during hierarchical modulation are described with reference to FIG. 4 by taking 16 QAM as an example. As illustrated in FIG. 4, in each 16 QAM constellation point, the MSBs are occupied by the second user equipment UE2 with poor channel quality, and the LSBs are occupied by the first user equipment UE1 with good channel quality. During demodulation, the second user equipment UE2 can obtain two bits (i.e., MSBs) of each constellation point by demodulation, and the first user equipment UE1 can obtain all four bits of each constellation point by demodulation.
In the future, the wireless communication system will use multi-antenna as an important means to improve validity and reliability of information. An important technology among multi-antenna technologies is a precoding technology of which a basic idea is that the user equipment computes an appropriate precoding matrix according to an channel obtained by estimation, and feeds back information of the precoding matrix to the base station. The base station will precode the transmitted data according to the precoding matrix, in the next time of scheduling the user equipment.
FIGS. 5A and 5B illustrate typical structural diagrams of conventional multi-antenna precoding transmission and diversity transmission, respectively. The structural diagram of precoding transmission is illustrated in FIG. 5A, wherein a first source data to be transmitted to a first user equipment UE1 is firstly encoded and rate-matched. Herein the encoding is ⅓ base encoding, and a sequence output by the rate matching is a sequence obtained through truncation or punch of the sequence output after the base encoding. The sequence length is related to modulation mode and the number of the transmission layers to be used. The structural diagram of diversity transmission is illustrated in FIG. 5B. Similar to FIG. 5A, a second source data to be transmitted to the second user equipment UE2 is firstly encoded and rate-matched. In order to be distinguished from FIG. 5A and facilitate the descriptions, the rate-matching used for the first user equipment UE1 is called as a first rate-matching, and the rate-matching used for the second user equipment UE2 is called as a second rate-matching. The symbol sequence output after the second rate-matching is a symbol sequence x. The symbol sequence x is subject to a constellation point mapping, and then is diversity transmitted.
In case hierarchical modulation is adopted in the multi-antenna precoding system, the following problems will occur for example when two user equipments are selected to perform hierarchical modulation:
1. when the two selected user equipments are configured in the precoding transmission mode in advance, a precoding matrix fed back by only one of the user equipments can be selected during the downlink data precoding, and the precoding matrix is not certainly suitable to another user equipment; and
2. when the precoding matrix adopted in the downlink data precoding is that fed back by one of the user equipments, another user equipment cannot decode the transmitted data because it does not know the precoding matrix.
Therefore, in the related art, there is a need of a method and apparatus adopting hierarchical modulation in the multi-antenna system.
Literatures cited herein are listed as follows, and they are incorporated herein by reference as if they were described herein.
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To be noted, the above descriptions of the prior art are just made for the convenience of clearly and completely describing the technical solutions of the present invention, and facilitating a person skilled in the art to understand. It shall not be deemed that these solutions are known to a person skilled in the art just because they are described in the Background section.